This invention relates to a steel oil ring assembly used in an internal combustion engine. More particularly, the invention relates to improvements in the shape of the abuting end portions of a spacer-expander having a corrugated shape in the radial as well as in the axial direction.
A spacer expander 10 of the type shown in FIG. 1(a) often is used in a steel oil ring assembly of the abovementioned kind. The spacer expander 10 (hereinafter referred to simply as a "spacer") has corrugations in the radial direction and is embraced from above and below by an upper rail and a lower rail.
As shown in FIG. 1(a), the spacer 10 in a conventional steel oil ring assembly has a continuous corrugated shape in the radial direction and is formed to have an overall annular shape as seen from the axial direction. The spacer 10 is split at one location on the circumference thereof, and the split ends of the spacer 10 meet at a butt portion 11. When the spacer 10 is fitted into a separate piston and compressed by the inner wall of a cylinder, the split ends at the butt portion 11 come into pressured contact with each other in such a manner that a force point (junction point) a occupies a position considerably above a center line X-X' of the height of the spacer corrugations. As a result, the state of contact and the contact force between upper- and lower-rail abutting portions 12, 12' which project from the vicinity of the force point a and the inner peripheral surfaces of the respective upper and lower rails become non-uniform. Owing to the influence of this non-uniformity, the outer peripheral surfaces of the upper and lower rails present at the butt portion 11 assume a state in which they do not contact the cylinder wall surface correctly. This causes a reduction in the oil scraping effect of the oil ring.
Another example of a spacer according to the prior art is shown in FIG. 1(b).
As shown in FIG. 1(b), a spacer expander 110 has a corrugated shape in the axial direction and is also embraced from above and below by an upper rail and a lower rail.
The spacer 110 in this conventional steel oil ring assembly has a continuous corrugated shape in the axial direction and is formed to have an overall annular shape as seen from the axial direction. The spacer 110 is split at one location on the circumference thereof, and the split ends of the spacer 110 meet at a butt portion 111. When the spacer 110 is fitted into a piston and compressed by the inner wall of a cylinder, the split ends at the butt portion 111 come into pressured contact with each other in such a manner that the force point (juntion point) a occupies a position considerably above the center line X-X' of the height of the spacer corrugations. As a result, the state of contact and the contact force between upper- and lower-rail abutting portions 112, 112' which project from the vicinity of the force point a and the inner peripheral surfaces of the respective upper and lower rails become non-uniform. Owing to the influence of this non-uniformity, the outer peripheral surfaces of the upper and lower rails present at the butt portion 111 assume a state in which they do not contact the cylinder wall surface correctly. This causes a reduction in the oil scraping effect of the oil ring.